1. Field of the Invention
The present invention relates generally to digital telecommunication systems having controllable ringing signal or other signal generation capability for interconnection to a plurality of analog lines, and specifically to telephone systems having improved ringing circuitry.
2. Description of the Prior Art
In telephone switching systems of the prior art, ringing has been provided from service circuits; i.e., ringing trunk lines such that at any given instant in time only one analog subscriber line may be rung from one such ringing circuit. Thus, a ring-trip circuit must be provided for each such prior art ringing circuit. When a requirement exists for immediate ringing, then the service circuit has had two sources of ringing connected thereto. One source being continuous and the other being phased. Such service circuit has included control which connects the analog subscriber line to be rung to the continuous ringing source for a predetermined time duration, such as 300 milliseconds, and then to the phased ringing source. Such prior art technique and similar known prior art ringing techniques require for multi-frequency ringing either different service circuits or the service circuits having the capability of providing any type of ringing. Such requirement involves the busing of different frequency sources and the inclusion of a plurality of relays or switches for each service circuit. The aforementioned ringing schemes of the prior art are unsuitable for use in digital switching systems, since ringing signals cannot be transmitted through the digital network.
Examples of such prior art telephone ringing systems may be had with reference to U.S. Pat. No. 3,767,857 relating to distributed immediate ringing circuits for a telephone system. U.S. Pat. No. 3,678,208 relating to immediate ringing by use of junctors assigned to different time slots of the ringing cycle. U.S. Pat. No. 4,075,430 (assigned to the assignee of the present invention) relating to line signaling over common highway for telecommunications systems. U.S. Pat. No. 3,005,053 relating to telephone signaling systems applying different signaling frequencies in different time slots. U.S. Pat. No. 3,085,133 relating to automatically controlled ringing with the provision of a ring immediately after connection, then ringing in accordance with the regular ringing cycle; and U.S. Pat. No. 3,118,019 relating to the connection of a ringing source to the desired telephone subscriber through a pair of gates in series, which must both switch in phase to permit the ringing signal to reach the subscriber. Digital telephone line circuits of the type contemplated to be employed with the present invention and which contain programmable signal generators having the capability of digitally generating ac and dc signaling voltages are available as exemplified, for example, by U.S. Pat. No. 4,161,633 of Robert Treiber and assigned to the assignee of present invention; and reference may be made to U.S. Pat. No. 4,161,633 for details of the implementation of programmable signal generation in a digital telephone line circuit. Digital telecommunications switching systems of the type with which the present invention is particularly advantageously employed are described in detail by U.S. Pat. No. 4,201,889 of A. J. Lawrence, et al, assigned to the assignee of the present invention, and reference may be made to said Lawrence et al Patent for details of such digital switching system.